The present invention relates to an internally-cooled desiccant absorber for use in a liquid-desiccant air conditioner and, more particularly, such an air conditioner having a high thermal coefficient of performance (COP).
The primary function of the absorber in a liquid-desiccant air conditioner is to dehumidify the supply air to the building. This is done most simply in a packed-bed absorber, which consists of a porous bed of contact medium that is flooded with desiccant. As the desiccant flows down through the bed, it comes in contact with the water-containing process air that can be flowing either down, up, or across the bed. The desiccant--which, by definition, has a strong affinity for water vapor--absorbs moisture from the process air.
During dehumidification, heat is released as the water vapor condenses and mixes with the desiccant. This heat will equal the latent heat of condensation for water plus the chemical heat of mixing between the desiccant and water. At desiccant concentrations typical of a liquid-desiccant air conditioner, the chemical heat of mixing will be about an order of magnitude smaller than the latent heat of condensation.
For the simple packed-bed absorber just described, the heat released during dehumidification will raise the temperature of the air and desiccant as they flow through the absorber. If the desiccant that flows off the bottom of the bed is not cooled before it is recirculated to the top spray nozzles, the air will leave the absorber at close to the same enthalpy as it entered. For example, air enters the absorber at 80.degree. F. [27.degree. C.], 50% R.H. (31.3 BTU/lb enthalpy) and leaves at 97.degree. F. [36.degree. C.], 20% R.H. (31.5 BTU/lb enthalpy). In this configuration, the absorber is a dehumidifier.
The preceding dehumidifier may be incorporated into an air-cooling system by cooling the desiccant before it is sprayed onto the absorber. This can be done by cooling the desiccant with externally chilled cooling water or other refrigerant in a heat exchanger. If the desiccant is cooled before it is sprayed onto the absorber bed, the air will leave the absorber at a lower enthalpy than it entered, i.e.., net cooling. An indirect evaporative cooler can further cool the process air without increasing its absolute humidity.
The preceding cooling/dehumidifying system can be made more compact by integrating the desiccant cooler into the absorber bed to provide an internally cooled absorber for a liquid-desiccant air conditioner. This internal desiccant cooler will most commonly be configured as a heat exchanger with liquid desiccant and process air flowing through one set of channels and a coolant flowing through the other set of channels. The coolant can be (a) chilled water, (b) a chilled refrigerant such as a water/glycol mixture, or (c) a boiling refrigerant, such as R12 or R22, or their HFC and HCFC replacements, that is supplied from a mechanical refrigeration system. The coolant can also be a film of flowing water in contact with an air flow that evaporatively cools the water. One embodiment of this last approach to an internally cooled absorber is the "three way" absorber described by Lowenstein, A., J. Marsala, M. Spatz, S. Feldman, and J. Tandler (1988) "Integrated gas-fired desiccant dehumidification vapor compression cooling system for residential application," Phase I Final Report, GRI-88/0326. This report available from the National Technical Information Service as Report No. PB89140842 at a nominal cost. In this report, the nominal operating conditions of the absorber are presented as 500 cfm for the process side and 750 cfm for the cooling side (page 72, first full paragraph). The absorber is described (page 69, third full paragraph) as being composed of two air-to-air heat exchangers that are each 30 cm.times.30 cm.times.60 cm. FIG. 4.21 (page 68) shows how the two heat exchangers are stacked within the absorber's cabinet. The discussion (page 69, first two full paragraphs) explains that the process air flows upward and the cooling air flows crosswise. In this configuration, the face area for the process flow is 30 cm.times.60 cm, and for the cooling flow, 60 cm.times.60 cm. Since only half the face area is available for air flow, the actual flow areas are 0.97 ft.sup.2 and 1.94 ft.sup.2 for the process and cooling air, respectively. The air velocities are, therefore, calculated to be about 500 fpm and 400 fpm for the process end cooling air. These values are the lower limits of the gas flow rates.
In the "three-way" absorbers, the internally-cooled absorber is constructed as a cross-flow parallel-plate heat exchanger. Process air flows through one set of passageways and cooling air through the other. On the process side, desiccant is sprayed onto the plates; on the cooling side, water is sprayed onto the plates. As the process air is dried in the "three-way" absorber, the heat that is released is immediately transferred to the cooling air. Since the cooling air is maintained at close to its wet-bulb temperature, the temperature rise for the desiccant as it flows through the absorber is very low. This increases the dehumidification capacity of the absorber.
An internally-cooled absorber is more difficult to fabricate than an absorber that separates the dehumidification and cooling processes--i.e., an absorber that uses simple contact media, coupled with either an air cooler or desiccant cooler. The most significant problem will be leakage between the desiccant and cooling water. However, the performance benefits provided by this absorber typically justify its use.
The relative flow rates of air and desiccant will have an important effect on the dehumidification performance of the absorber.
Accordingly, it is an object of the present invention to provide an air conditioning process wherein the internally-cooled absorber is operated at the optimum flow rates of liquid desiccant and cooling water in order to maximize the coefficient of performance (COP) of the air conditioner.